This invention relates to a vitreous enamel steel sheet excellent in enameling properties (bubbling and black spot resistance and adhesiveness) and formability and to a method of producing the same, and is characterized especially by producing the same through continuous casting.
Vitreous enamel steel sheets have conventionally been produced by ingot casting into capped steel or rimmed steel, primary rolling, hot rolling, cold rolling and, then, decreasing carbon and nitrogen concentrations to several tens of ppm or less through decarbonization by open coil annealing and, in addition, denitrification annealing. The vitreous enamel steel sheets produced following these process steps have, however, a problem of a high manufacturing cost because the ingot casting and primary rolling process is included in the manufacturing steps and the decarbonization/denitrification annealing is required. Another problem is that they are not applicable to final products requiring intensive deep drawing work.
Facing these problems, some vitreous enamel steel sheets produced by continuous casting have been proposed for the purpose of cutting production costs. For example, vitreous enamel steel sheets produced by continuous casting of high oxygen steels, such as the one disclosed in Japanese Examined Patent Publication No. S57-49089, have excellent enameling properties. However, they still have problems that they cannot be used for deep drawing applications due to their poor formability and that they still require decarbonization annealing or decarbonization/denitrification annealing by the open coil method in relation to aging properties.
As a means to overcome the above problems, Japanese Unexamined Patent Publication No. S59-190331 discloses an ultra-low carbon, ultra-low nitrogen vitreous enamel steel sheet produced by continuous casting. The proposed technology consists of controlling concentrations of C and N of a high oxygen steel to an extremely low level of C less than 30 ppm or C+N less than 30 ppm in steelmaking processes. The reduction of the C and N concentrations in steel to such a low level, however, inevitably incurs an increase in steelmaking costs and, what is more, even if C+N less than 30 ppm is achieved, perfectly non-aging steel sheets cannot be obtained.
Some methods to achieve deep drawability and non-aging properties through the addition of Ti or Ti+Rem have been disclosed as countermeasures against the above problem. The technologies disclosed in Japanese Unexamined Patent Publications No. S51-32417 and No. S52-128822 are such examples. The former is a method to enhance deep drawability and obtain non-aging properties by eliminating C and N in solid solution through addition of Ti+Rem to an ultra-low carbon steel and, in parallel, to improve fish scale resistance by forming (Ti, Rem)S by means of S added in a large quantity. By this method, good vitreous enameling properties can be obtained in two-coat enameling in which a strongly adhesive ground coat is used, but bubbling occurs in one-coat enameling and fish scale resistance is insufficient. The method has another problem of a low product yield due to frequent surface defects caused by the addition of Rem. The latter is a method to improve deep drawability by means of a Ti-added ultra-low carbon steel and adhesiveness in two-coat enameling by Cu addition, but it has a problem in that it cannot be applied to steel sheets for one-coat enameling.
In the above situation, after a series of arduous researches, the inventors of the present invention submitted Japanese Patent Application No. H9-274932 regarding a vitreous enamel steel sheet excellent in fish scale resistance and deep drawability and a method to produce the same, proposing a high oxygen vitreous enamel steel sheet containing Nb and V and produced by continuous casting. The proposed technology could provide a steel sheet excellent in deep drawability, but there was a problem that its bubbling and black spot resistance was inferior to totally Al-free high oxygen steels.
The object of the present invention is to overcome the above problems of conventional steel sheets for vitreous enamel use and to provide a deep-drawable and non-aging vitreous enamel steel sheet produced by continuous casting and excellent in bubbling and black spot resistance and adhesiveness in one-coat enameling, and a method to produce the same.
The present invention has been established as a result of wide-ranged studies for the purpose of overcoming the shortcomings of the conventional steel sheets and their production methods. The findings {circle around (1)} to {circle around (6)} described below were reached as a result of examinations of the influences of the following chemical composition and production conditions on formability and enameling properties of vitreous enamel steel sheets.
Chemical composition (in mass %):
0.0005 to 0.0025% of C, 0.05 to 0.35% of Mn, 0.015 to 0.07% of O, 0.005 to 0.06% of Nb, 0.03 to 0.07% of V, 0.05% or less of Cu, 0.05% or less of Si, 0.005 to 0.025% of P, 0.035% or less of S, and 0.0015 to 0.0035% of N.
Production conditions:
A reheating temperature of 1,250 to 1,050xc2x0 C., a hot rolling finishing temperature of 750 to 950xc2x0 C., a coiling temperature of 500 to 800xc2x0 C., a cold reduction ratio of 50% or more, and an annealing at 650 to 850xc2x0 C. for 1 to 300 min.
Enameling properties:
Fish scale resistance, bubbling and black spot resistance, surface defects and adhesiveness were examined regarding steel sheets coated with an enamel film of 100 xcexcm in thickness by a one-coat enamel treatment after a pickling and a Ni treatment.
{circle around (1)} The lower the amounts of C and oxygen, the better the deep drawability.
{circle around (2)} A deep drawability of r-value  greater than 2.0 is attained by reducing C content to 0.002% or less and adding Nb to 0.03% or more.
{circle around (3)} An aging index of 5 MPa or less is obtained regardless of annealing conditions, when the following conditions are satisfied: C less than 0.002%, Vxe2x89xa70.03%, and Nbxe2x89xa70.03%.
{circle around (4)} A good enamel adhesiveness is obtained when the following inequality is satisfied:
0.2 greater than T. Mnxe2x88x922.0(O)+0.8(V)+0.5(Nb).
{circle around (5)} Bubbling and black spots do not occur on enameled steel sheets also when the following inequality is satisfied:
0.2 greater than T. Mnxe2x88x922.0(O)+0.8(V)+0.5(Nb).
{circle around (6)} Hydrogen permeation time, which has good correlation with fish scale resistance, is influenced by the contents of oxygen, Mn, V and Nb, and the larger the contents of these elements, the longer the hydrogen permeation time.
The gist of the present invention, which was established based on the above facts, is as follows:
(1) A vitreous enamel steel sheet produced by continuous casting, excellent in formability, bubbling and black spot resistance and enamel adhesiveness, comprising, in mass %, 0.002% or less of C, 0.05 to 0.2% of Mn, 0.01% or less of Si, 0.004% or less of N, 0.015 to 0.05% of 0, below 0.01% of P, 0.025% or less of S, 0.02 to 0.04% of Cu, 0.03 to 0.05% of Nb, and 0.03 to 0.07% of V, satisfying the inequality 0.2 greater than Mn (%)xe2x88x922.0xc2x7O (%)+0.8xc2x7V (%)+0.5xc2x7Nb (%), and the balance consisting of Fe and unavoidable impurities.
(2) A method to produce a vitreous enamel steel sheet produced by continuous casting, excellent in formability, bubbling and black spot resistance and enamel adhesiveness, characterized by hot rolling a continuously cast slab, comprising, in mass %, 0.002% or less of C, 0.05 to 0.2% of Mn, 0.01% or less of Si, 0.004% or less of N, 0.015 to 0.05% of O, below 0.01% of P, 0.025% or less of S, 0.02 to 0.04% of Cu, 0.03 to 0.05% of Nb, and 0.03 to 0.07% of V, satisfying the inequality 0.2 greater than Mn (%)xe2x88x922.0xc2x7O (%)+0.8xc2x7V (%)+0.5xc2x7Nb (%), and the balance consisting of Fe and unavoidable impurities, at a finishing temperature of 800xc2x0 C. or higher and a coiling temperature of 600 to 800xc2x0 C., cold rolling the hot rolled strip at a reduction ratio of 60% or more, and then annealing the cold rolled strip at a recrystallization temperature or higher.